在xcode 6.1中打开storyboard文件时，mac变为黑色

I have problem in Xcode. I got black screen when I load storyboard of any project project. It was working fine before but suddenly when i changed background of one storyboard file, it started turning the screen totally black. MAC OS Mavericks, xcode 6.1. Please tell me cause and way for solving this problem! Thank you!I have problem in Xcode. I got black screen whe